The present invention generally relates to a demodulator and more particularly, to an orthogonal amplitude modulation wave demodulator. There are band-pass filters for a fixed equalizer to be employed in said orthogonal amplitude modulation wave demodulator.
In the present specification, a phase modulation wave is also considered to be one form of the quadrature amplitude modulation wave. Accordingly, the fixed equalizer and the quadrature modulated wave demodulator can also be utilized for the demodulation of the phase demodulation waves.
In demodulators to be used in communications for example, communications that are transmitted over wire or communication withouth wire (i.e. wireless), a fixed equalizer having a characteristic opposite to that of a communication line is incorporated in order to remove distortion produced by the characteristics inherent in the communication line.
A schematic block diagram for one example of conventional quadrature amplitude modulation wave demodulator is shown in FIG. 3. In FIG. 3 a digital signal processing technique is utilized, and this is normally realized by a digital signal processor (referred to as a DSP hereinafter).
In FIG. 3, the quadrature amplitude modulation wave demodulator includes a multiplication section for affecting multiplication between a diqital reoeption signal inputted to an input terminal 1 and two kinds of reference carrier waves in quadrature to each other. There is a filter section for effecting removal of a frequency component two times that of the carrier waves, and waveform shaping, with respect to the output of said multiplication section, and a low-pass filter type fixed equalizer for equalization of the communication line.
The multiplication section is provided with a reference carrier wave generator 2, a .pi./2 phase shifter 3, and two multipliers 4 and 5, the outputs of which are respectively applied to reception low-pass filters 6 and 7. The low-pass filters 6 and 7 normally having the same characteristics perform the removal of their frequency component two times that of the carrier waves produced by the multiplication between the input signal and reference carrier wave, and also perform the function of waveform shaping. The low-pass filter type fixed equalizer is provided with four low-pass filters 8,9,10 and 11, and adders 14 and 15. The low-pass filters 8 and 10 have the same characteristics , and the low-pass filters 9 and 11 also have the same characteristics. In this fixed equalizer, the vector calculation is executed for equalization of the circuit line with respect to a pair of outputs of the filter section. At the output terminals 16 and 17, the same component and the quadrature component as the result of demodulation are respectively outputted.
The low-pass filters 6 and 7 and the low pass filters 8,9 10 and 11 for the fixed equalizer are each normally realized by a finite impulse response type (referred to as FIR type hereinafter) digital filter, whose general construction is illustrated in FIG. 4. As shown in FIG .4, such FIR type digital filter includes gain tap factor memories 41, delay registers 42, multipliers 43, and an adder 44 connected to each other as illustrated.
Referring back to Fig.3, the conventional quadrature amplitude modulation wave demodulator includes six filters (i.e. the low-pass filters 6 and 7, and the low-pass filters 8,9, 10, and 11 for the equalization of the circuit lines). The calculating to be executed within the filter includes many multiplications and additions. Where the demodulator is to be used for example, by a DSP, a large load is applied to the DSP, thus requiring a high speed and large capacity DSP, which is naturally expensive.